If the item was listed as a postage item when the buyer purchased this, then they will not receive any collection code to give to you, and this is your only proof the item was purchased.
The best way forward is to cancel the item and relist it as a Collection only item, then after buying and paying the buyer will receive this important 6 digit QR code. Explain they must bring this with them on collection as the item will not be released without it.
eBay read all messages, be clear when you message the buyer the reason for cancelling the item that this is not for an off eBay sale, it will be relisted, for the buyer to buy again, from your eBay listing.
@milis1969